While tactile acuity for pressure has been extensively investigated, far less is known about acuity for vibrotactile stimulation. Vibrotactile acuity is important however, as such stimulation is used in many applications, including sensory substitution devices. We tested discrimination of vibrotactile stimulation from eccentric rotating mass motors with in-plane vibration. In 3 experiments, we tested gradually decreasing center-to-center (c/c) distances from 30 mm (experiment 1) to 13 mm (experiment 3). Observers judged whether a second vibrating stimulator (‘tactor’) was to the left or right or in the same place as a first one that came on 250 ms before the onset of the second (with a 50-ms inter-stimulus interval). The results show that while accuracy tends to decrease the closer the tactors are, discrimination accuracy is still well above chance for the smallest distance, which places the threshold for vibrotactile stimulation well below 13 mm, which is lower than recent estimates. The results cast new light on vibrotactile sensitivity and can furthermore be of use in the design of devices that convey information through vibrotactile stimulation.

Relative vibrotactile spatial acuity of the torso

Relative vibrotactile spatial acuity of the torso
Ómar I. Jóhannesson 0 1 2
Rebekka Hoffmann 0 1 2
Vigdís Vala Valgeirsdóttir 0 1 2
Rúnar Unnþórsson 0 1 2
Alin Moldoveanu 0 1 2
Árni Kristjánsson 0 1 2
0 Faculty of Automatic Control and Computers, Polytechnic University of Bucharest , Bucharest , Romania
1 Faculty of Industrial Engineering , Mechanical Engineering and Computer Science , University of Iceland , Reykjavik , Iceland
2 Faculty of Psychology, School of Health Sciences, University of Iceland , Reykjavik , Iceland
3 Rebekka Hoffmann
While tactile acuity for pressure has been extensively investigated, far less is known about acuity for vibrotactile stimulation. Vibrotactile acuity is important however, as such stimulation is used in many applications, including sensory substitution devices. We tested discrimination of vibrotactile stimulation from eccentric rotating mass motors with in-plane vibration. In 3 experiments, we tested gradually decreasing center-to-center (c/c) distances from 30 mm (experiment 1) to 13 mm (experiment 3). Observers judged whether a second vibrating stimulator ('tactor') was to the left or right or in the same place as a first one that came on 250 ms before the onset of the second (with a 50-ms interstimulus interval). The results show that while accuracy tends to decrease the closer the tactors are, discrimination accuracy is still well above chance for the smallest distance, which places the threshold for vibrotactile stimulation well below 13 mm, which is lower than recent estimates. The results cast new light on vibrotactile sensitivity and can furthermore be of use in the design of devices that convey information through vibrotactile stimulation.
Vibrotactile acuity; Vibrotactile accuracy; Discrimination of vibrotactile stimulation; Sensory substitution
-
Ómar I. Jóhannesson and Rebekka Hoffmann contributed equally
to the manuscript.
Introduction
Tactile communication systems have been developed for a
wide range of applications such as for navigation, alerting,
and sensory substitution devices (SSDs) for people with
visual and auditory impairments
(Ranjbar et al. 2009; Yuan
et al. 2005; Sampaio et al. 2001)
. The aim with SSDs is to
assist sensory impaired people by partially restoring
function with input from other senses. Touch or audition, for
example, can be used to convey information that is
otherwise not available, such as in vision loss (
Maidenbaum et al.
2014
;
Jóhannesson et al. 2016
).
SSDs have been developed for various body parts [see
Krist
jánsson et al. (2016
),
Jóhannesson et al. (2016
), for
reviews], such as the tongue
(Tang and Beebe 2006; Chebat
et al. 2011)
, hands (Ito et al. 2005), and fingertips
(Koo et al.
2008)
. Applications that stimulate passive body parts (torso,
arm) may be most practical however, since the tongue and
hands should preferably be available for other use
(Dakopoulos and Bourbakis 2010; Kristjánsson et al. 2016)
.
Disadvantages of passive areas, such as the reduced
sensitivity of hairy compared to glabrous skin
(Bolanowski et al.
1994; Sofia and Jones 2013)
, as well as the reduced spatial
acuity of these areas compared to active body parts
(Weinstein 1968; Johnson and Phillips 1981)
, can be compensated
for by the larger skin area that can be used for conveying
information.
In a pioneering study
Bach-Y-Rita et al. (1969
) introduced
a dental chair with a 20 × 20 array of tactors1 that
translated visual information obtained by a camera into tactile
stimulation on the participant’s back. Following extensive
training (10–20 h, or even up to 150 h), participants were
able to recognize objects, and some highly trained observers
were able to report remarkable detail, such as whether
people wore glasses or not. Since then, various mobile, tactile
stimulation devices have been developed, in which vibration
is presented to participants based on information from video
cameras
(McDaniel et al. 2008; Cosgun et al. 2014; Johnson
and Higgins 2006)
or a GPS receiver
(Van Erp et al. 2005b)
.
For instance, Van Erp et al. (2005b) developed a tactile belt
with 8 tactors coding distance by vibration frequency and
direction by vibration location. Collectively, these results
suggest that tactile belts can be effective communication
systems, especially for navigation.
The widespread use of mobile devices and wearable
computers with limited screen space for visual information has
led to increased interest in tactile communication systems.
Vibrotactile equipment has become more accessible,
affordable, as well as more sophisticated and less intrusive
resulting in more effective and user-friendly designs
(Jones and
Sarter 2008)
.
Increased use of advanced tactile equipment highlights
the importance of psychophysical investigations of basic
mechanisms underlying human tactile perception. One of
the main challenges in designing tactile displays is
determining what type of information can be presented tactually and
which parameters of stimulation can be used to effectively
convey information. One issue involves empirical
assessment of the maximal throughput of the skin. How many
tactors can be arranged on the torso and how densely can
they be placed before their loci become indistinguishable?
Determining this threshold is important for the design of
tactile displays since the more tactors can be placed within
a defined area, the more information can be conveyed in
space, which eliminates the need for encoding information
by deploying the dimension of time. Psychophysical studies
on tactile spatial acuity of different body sites can provide
the required empirical basis for determining the optimal
stimulation type for tactile displays.
Tactile spatial acuity
In the nineteenth century
Weber (1834)
performed
pioneering psychophysical research on relative spatial acuity
of the skin, introducing two measurement methods that
are still in use today. One is the two-point threshold (2PT),
which is measured by presenting either one stimulus or two
1 A tactor, short for tactile vibrator, is a motor that creates vibrations.
simultaneous stimuli while the distance between them is
steadily decreased, to assess the threshold of when two
stimuli are erroneously perceived as one. Another method is the
point of localization (PL), which is assessed by successively
presenting two tactile stimuli so that the second stimulus is
either in the same location or increasingly distant from the
first, in order to determine the threshold when two
stimulation sites are correctly perceived as two.
Weinstein (1968)
used both methods to study the sensitivity of a large number
of body parts and found that spatial tactile acuity with the
two-point threshold was 2–4 times lower than with point
localization although both thresholds were highly correlated
over body parts. The lowest thresholds were found on the
fingertips (2.5 and 1.5 mm, for PL and 2PT, respectively),
whereas thresholds for the back were ca. 40 and 10 mm,
respectively.
Although later studies have essentially confirmed
Weinstein’s acuity map
(Johnson and Phillips 1981; Stevens and
Choo 1996)
, some argue that Weinstein’s methods
underestimate the skin’s actual spatial acuity and have explored other
assessment methods. Vierck and Jones (1969) measured
discrimination of the size of plastic cylinders impressed on the
forearm (on the palm side) concluding that the
discrimination threshold is between 2 and 6 mm on the forearm.
Jones
and Vierck (1973)
tested discrimination of the length of two
straight lines impressed on the forearm, ranging in length
from 1.6 to 127 mm. The average discrimination threshold
was 21 mm or about two times lower than the 2PT
threshold. This suggests that acuity can be higher than Weinstein’s
results indicate, especially if patterns, rather than points are
used for stimulation
(Bach-y-Rita and Kercel 2003; Gibson
1962; Loomis et al. 2012)
.
Vibrotactile spatial acuity
While tactile spatial acuity for pressure has been explored
extensively, most applications nowadays use vibrotactile
stimuli. All skin receptors, except the Ruffini endings,
respond to vibrating stimuli but their sensitivity and
frequency range differ substantially. The Pacinian corpuscles
have the largest frequency range (5–1000 Hz) with peak
sensitivity at 200 Hz. Furthermore, they do not respond to
indentions of the skin
(Gardner and Johnson 2013)
. A layer
(e.g., a T-shirt) between the tactors and the skin can
substantially reduce effects of lateral motion, stretching of the skin
and the effects of edges and points which are the preferred
stimuli of Meissner corpuscles, Ruffini endings and Merkel
cell receptors, respectively. Generalizing threshold measures
obtained with static pressure to vibratory stimuli may
therefore be misleading. Furthermore, the size of the contact area
may also influence the ability to localize vibrotactile stimuli,
but this has apparently not been systematically investigated.
One way of determining the optimal spacing and number
of vibrating tactors in tactile displays is to assess absolute
spatial acuity. The method for measuring absolute spatial
acuity [for example by
Cholewiak et al. (2004
),
Lindeman
and Yanagida (2003)
] is to present one vibrotactile stimulus
within a display of tactors with fixed distances and ask the
participants to indicate the location of the activated tactor on
an isomorphic keyboard or screen.
Lindeman and Yanagida
(2003)
tested absolute localization accuracy on the back of
participants by using a 3 by 3 array of tactors. They found the
accuracy to be high (around 84%) for an inter-tactor distance
of 60 mm. However, the threshold of vibrotactile spatial
acuity, the minimum detectable distance between tactors, cannot
be inferred from their results.
Cholewiak et al. (2004
) tested
absolute localization accuracy on the abdomen by varying the
number of tactors in an array in three conditions (12, 8, and 6
tactors). Localization accuracy was highest around the navel
and the spine and accuracy increased with fewer tactors and
longer distances. The inter-tactor distance was not constant
between participants, however, since the tactors were mounted
on an elastic band, which the participants wore around the
torso and therefore stretched depending on their girth,
resulting in different spacing between tactors for each participant.
Their results, therefore, do not allow assessment of the
minimum detectable inter-tactor distance.
Taken together, even though measurements of absolute
localization accuracy can contribute to the improved design
of tactile displays, the method is not suitable for
determining thresholds for vibrotactile spatial acuity. A number of
studies
(Eskildsen et al. 1969; van Erp 2005a; Novich and
Eagleman 2015)
therefore follow a different approach,
measuring relative spatial acuity by applying point localization
methods (PL, Weinstein 1968).
Jones (2011)
discusses the
two approaches, stating that limitations of the former do
not appear to apply to the latter. However, the results of
the few studies measuring relative vibrotactile spatial
acuity have been inconsistent. Eskildsen et al. (1969) tested the
relative position of tactors, using a row of 5 mechanically
vibrating (60 Hz) tactors mounted in the back of a dentist’s
chair. Seven stimulus distances were tested, ranging from 0
to 60 mm in steps of 10 mm, resulting in mean thresholds
of 11 and 10 mm for simultaneous and successive
presentation, respectively. Also, van Erp (2005a) tested
horizontal spatial acuity using an array of 14 tactors on one
occasion and an array of 11 tactors on another (in both cases
at 250 Hz) where the distance between the centers of the
tactors was 20 mm. Unfortunately, van Erp did not report the
absolute accuracy ratio but with a fitting procedure they
estimated that the discrimination threshold was between 20 and
30 mm, and approximately 10 mm around the navel and the
spine. Van Erp (2005a) also tested vertical acuity,
concluding that it was similar to horizontal acuity. Recently, Novich
and Eagleman (2015, experiment 2) reported surprisingly
low tactile sensitivity for an array of 5 × 2 tactors on their
participant’s backs finding that the tactors needed to be at
least 60 mm apart for two independent vibrating stimuli to
be discriminable at more than 80% correct, regardless of
stimulus type (e.g., spatiotemporal “sweeps” versus single
vibratory pulses).
Current goals
Our aim was to systematically investigate the relative
spatial acuity on the torso to vibrotactile stimulation with the
ultimate goal of using the results to formulate guidelines
for inter-tactor spacing when designing tactile displays with
vibrotactile actuators. We report the results of 3 experiments
where vibrotactile spatial acuity was assessed with
“vibrosponge” devices where 9 tactors (in a 3 by 3 array) were
mounted on foam material and strapped to observers’ torsos,
and with a tactile vest with 64 tactors in an 8 by 8 array.
Methods
Three experiments were conducted to systematically
investigate the relative vibrotactile spatial acuity of the torso using
two types of stimulation devices, a “vibro-sponge” and a
tactile vest.2 The center-to-center (c/c) distance between the
tactors on the vest was fixed to 40 mm in all experiments.
The distance between the tactors on the vibro-sponge was
gradually decreased from 30 to 13 mm c/c in experiments
1, 2 and 3, respectively, towards the lower limit of possible
distance without inter-tactor contact. The general methods
used in all experiments are described below.
Participants
A full within-subjects design would have entailed advance
decisions on the minimal tested distance. Since the minimal
distance was what we were looking for this was not possible.
To compensate for this, each group of participants in the
different experiments also performed a task with the tactile
vest (see below) that was identical within-experiment, in an
effort to assess whether there were any major sensitivity
differences between the groups that could account for potential
differences in performance on the vibro-sponge tests.
Participants in experiment 1 were 10 (5 F, 5 M) aged
between 20 and 38 years (M = 28.6 years, SD = 5.3 years).
In experiment 2 there were 10 participants (5 F, 5 M) aged
from 20 to 31 years (M = 24.3 years, SD = 2.9 years) and
2 We ran a control experiment (Experiment 4) to compare
performance with the vest with 8 by 8 versus 3 by 3 tactors. Those results
are discussed at the end of the “Results” section.
in experiment 3 there were 10 participants (5 F, 5 M) aged
between 22 and 31 (M = 26 years, SD = 3.2 years). The
participants differed between the three samples with the
exception that half of the participants of experiment 2 also
participated in experiment 3. All participants were naïve
about the purpose of the study and were students or staff at
the University of Iceland, and gave written informed consent
before participating. All experiments were approved by the
National Bioethical Committee of Iceland (VSN-15-107).
Apparatus
In all experiments, we used both a tactile vest (40 mm c/c
distance, Fig. 1) and a vibro-sponge (30, 20 and 13 mm c/c
in experiments 1, 2 and 3, respectively, Fig. 2). The
purpose of this arrangement was to assess whether any changes
in accuracy with the vibro-sponge reflected individual
differences. If similar results are observed for the tactile vest
across the groups tested in the different experiments, this
supports the conclusion that any inter-tactor distance
differences reflect general acuity differences rather than individual
differences. Half the participants in each experiment started
with the vest and half with the vibro-sponge.
Tactors
All devices were equipped with eccentric rotating mass
(ERM) tactors (also called coin cell motors), with in-plane
vibration (i.e., the vibrations were parallel to the skin).
Custom software written in PsychoPy
(Peirce 2007, 2009)
was
used to control stimulus presentation by sending the relevant
command through the virtual serial port to a custom-built
electronic circuit that controlled the tactors. The diameter of
the tactors used on the vibro-sponge was 10 mm and their
weight was 0.9 g. The tactors were running on 5 V and the
frequency was 183 Hz at full speed (11,000 RPM). The
diameter of the tactors used on the tactile vest was 8 mm,
otherwise their specifications were similar to those used on
the vibro-sponge.
Tactile vest
The tactile vest consists of 64 (8 by 8) tactors (8 mm
diameter) that are mounted at a horizontal c/c distance of 40 mm,
and a vertical c/c distance of 52 mm on the back (see Fig. 1).
In experiments 1–3 all the tactors on the vest were used
(determined randomly for each trial) but in experiment 4,
which served as a control, we used 9 tactors (in a 3 by 3
grid) located at similar locations as the tactors on the
vibrosponge (see below). The aim of experiment 4 was to test
whether different tactor numbers between the vest and the
vibro-sponge affected performance. The size of the tactile
vest was individually adjustable (note that this did not affect
intertactile spacing) and participants wore it over their own
shirts.
The vibro-sponge was used for testing distances smaller than
40 mm c/c in experiments 1–3 since the inter-tactor distance
on the tactile vest was not adjustable. The vibro-sponge
consists of a 3 by 3 tactor array (see Fig. 2) and was placed
centrally on the participants’ back. The c/c distance between
the tactors (10 mm diameter) was 30 mm in experiment 1,
20 mm in experiment 2, and 13 mm in experiment 3. As
with the vest, the size of the vibro-sponge was individually
adjustable and participants wore it over their own shirts.
Procedure
In all experiments the task was a 3-alternative forced choice
(3AFC) task that involved judging whether the second tactor
that was activated, was to the left or right of the one
activated first or whether it was the same one. Each trial began
following a random interval between 1100 and 1700 ms in
100 ms steps. Participants used the left and right arrow keys
on a keyboard to judge the location of the second tactor
relative to the first one and the space bar if they thought that
the second tactor was at the same location as the first.
Importantly, participants wore headphones playing white noise
during the experiment to mask the sound of the motors,3
which could otherwise be a cue. The tactors were turned on
for 200 ms with a 50-ms delay between the offset of the first
tactor and the onset of the second. The location of the first
tactor, and whether the second tactor was to the left, right
or the same as the first was randomly determined. The total
3 Before running the main experiments, we ran a pilot experiment to
find the white noise level that adequately masked the sound from the
tactors.
parisons between these conditions. The accuracy never significantly
differed between experiments where the vest was used, while the
difference in RT between experiments 2 and 3 was significant
number of different tactor combinations for the tactile vest
was 144 and each combination was repeated 4 times
resulting in 576 trials (experiments 1, 2 and 3). The total number
of combinations using the 3 by 3 array with the vibro-sponge
(and the vest in experiment 4) was 9 and each combination
was presented 15 times resulting in 135 trials.
Statistical analyses
Before analyzing response times (RTs), trials with RTs that
deviated more than 3 standard deviations (SDs) from each
individual’s mean as well as trials where RTs were less than
100 ms and all trials with incorrect responses were removed.
We used R
(R Core Team 2015)
running in the RStudio
environment
(RStudio Team 2015)
for all analyses. To assess the
significance of any effects of distance and horizontal location
on accuracy and response times, we used repeated measures
ANOVAs
(aov; R Core Team 2015)
. In all experiments there
were three possible response types so that chance level was
0.33. One-sample t tests were used to assess whether accuracy
differed significantly from chance
(t test; R Core Team 2015)
and the Tukey’s honest significant difference test was used for
all post hoc comparisons
(Tukey HSD; R Core Team 2015)
.
tively). The error bars show 2× within-subjects’ SEMs and the
numbers on the lines denote the p values for post hoc comparisons
between these conditions
We also measured whether performance would improve with
repetition that might reflect practice or habituation.
Results
Tactile vest in experiments 1–3
Since the main purpose of using the tactile vest was to assess
any differences between the groups and to establish a
baseline, we therefore report the results of those tests together
in Fig. 3.
The average accuracy for the tactile vest in experiments
1, 2 and 3 was similar for the groups suggesting that
individual variation is unlikely to explain any effects of
different distances with the vibro-sponge. Average accuracy in
experiments 1, 2 and 3 ranged from 0.70 to 0.77, and always
significantly differed from chance (see Fig. 3a). Repetitions
did not increase accuracy in any of the experiments (all
ps > 0.2), and there were no differences between the
genders (all ps > 0.19).
Response times differed significantly between
experiments 2 and 3 which might be because half of the
participants in experiment 3 had previously participated in
experiment 2. None of the other comparisons revealed
significant differences (Fig. 3b). If anything, this may
suggest that the group in experiment 3 performed slightly
better than the others, but this conclusion is premature
since accuracy was comparable to accuracy for the groups
in experiments 1 and 2. RTs decreased as a function of
repetitions in experiments 1, 2 and 3 (see further
discussion below). In experiment 1 the average RT in the first
block was 751 ms (SD = 261 ms) and in the last block it
was 648 ms (SD = 222 ms). In the first block in
experiment 2 the average RT was 827 ms (SD = 292 ms) and in
the last block it was 671 ms (SD = 210 ms). In experiment
3 the average RT in block 1 was 620 ms (SD = 203 ms)
and decreased to 563 ms in block 4 (SD = 166 ms). This
decrease in response times with practice may reflect that
performance improved with practice for the tactile vest.
The most important result is, however, that accuracy was
comparable for the groups in the 3 experiments,
suggesting that there are little differences in tactile performance
between the 3 groups with identical stimuli.
Mean accuracy and response times are shown in Fig. 4 for
the 3 distances. The tactors on the vibro-sponge were in a
3 by 3 grid (3 columns and 3 rows). Therefore, the tactors
were within an area in which accuracy has been reported
to be higher than further away from the spine
(see e.g., van
Erp 2005a)
. Because of this, generalizing our results to areas
further away from the spine should be done with caution.
There was no difference in accuracy between the genders (all
ps > 0.58).
The aim with experiment 1 was to determine the
accuracy that can be achieved with an inter-tactor distance of
30 mm (c/c) with the vibro-sponge. Average accuracy was
0.91 (SD = 0.29) and differed significantly from chance
(0.33), t(9) = 37.91, p < 0.001. Performance was close
to ceiling, leaving little room for improvement, so the
effect of repetitions was not significant [F(14,126) = 1.06,
p = 0.401]. To ensure that there were no artifacts from
any potential differences between individual tactors, we
also tested whether there were any significant differences
between rows or columns. No such effects or interactions
were found (all ps > 0.24). After removing 130
incorrect responses (8.8% of the data) and 25 outliers (1.8% of
the remaining data), average response time was 620 ms
(SD = 209 ms). The main effect of repetitions was not
significant, F(14,126) = 1.25, p = 0.248. The main effect of
stimulation rows on RTs was significant [F(2,18) = 6.95,
p = 0.006], but a Tukey’s post hoc test showed that there
were no significant differences between the different rows
(all ps > 0.4).
Experiment 2 (20 mm)
In experiment 2 we tested accuracy with an inter-tactor
distance of 20 mm (c/c) with the vibro-sponge. The
average accuracy was 0.82 (SD = 0.39) and differed
significantly from chance; t(9) = 12.73, p < 0.001. Accuracy did
not change with repetitions; F(14,126) = 1.15, p = 0.325.
Again, we found neither significant main effects nor
interactions of stimulation rows and columns on accuracy (all
ps > 0.06). After removing 245 incorrect responses (18.1%
of the data) and 23 outliers (2.1% of the remaining data),
average response time was 746 ms (SD = 300 ms). The
effect of repetitions on response times was not significant
F(14,126) = 1.22, p = 0.270), and neither stimulation rows
nor columns nor the interaction between them affected
response times (all ps > 0.3).
Experiment 3 (13 mm)
In experiment 3, we measured accuracy with an inter-tactor
distance of 13 mm (c/c), using the vibro-sponge. This is the
smallest distance possible when using coin cell motors with
a diameter of 10 mm as they would collide if placed closer
to one another. Average accuracy was 0.64 (SD = 0.48)
and was significantly above chance, t(9) = 9.04, p < 0.001.
Repetitions did not increase accuracy; F(14,126) = 0.92,
p = 0.535. As before, neither main effects of rows and
columns on accuracy nor interactions between them were
significant (all ps > 0.12). After removing 492 incorrect
responses (36.4% of the data) and 18 outliers (2.1% of
the remaining data), average response time was 654 ms
(SD = 240 ms). The response times decreased as a function
of repetitions, F(14,126) = 1.78, p = 0.049. The mean RT
in block 1 was 749 ms (SD = 283 ms) and in block 15 it was
603 ms (SD = 207 ms). There were neither significant main
effects on RTs of rows and columns nor significant
interactions between them (all ps > 0.1).
Comparison of accuracy across experiments
To assess effects of inter-tactor spacing for spatial
vibrotactile acuity using the vibro-sponge, and in an attempt to
assess tactile acuity for such stimulation, we combined the
data from experiments 1, 2 and 3, comparing accuracy and
RTs between them. While we realize that the groups are not
fully comparable, and comparisons across experiments must
carry that caveat, we note, importantly, that the results for
the tactile vest revealed no differences between the groups
on a comparable task, decreasing the likelihood that group
differences explain the patterns. In fact, if anything, the RTs
suggest that participants in experiment 3 may have
performed slightly better than others with the tactile vest.
To assess effects of c/c distance on accuracy, we
compared accuracy rates between experiments 1, 2 and 3 (c/c 30,
20 and 13 mm, respectively). The main effect of experiment
was significant [F(2,18) = 19.26, p < 0.001]. Figure 4 shows
the post hoc comparisons. Accuracy was always well above
chance (0.33), even with the smallest inter-tactor distance of
13 mm c/c, ranging from 0.66 to 0.91 (all p values < 0.001).
In sum, the results suggest that the vibrotactile spatial acuity
of the torso is equal to or smaller than 13 mm since accuracy
is above chance even for this smallest c/c distance. But we
also note, importantly, that the accuracy rates significantly
decreased (p < 0.001) between 20 mm (experiment 2) and
13 mm (experiment 3) and between experiments 1 and 3
(p < 0.001). However, there was no significant difference
(p = 0.122) between the distances of 30 mm (experiment
1) and 20 mm (experiment 2), although the trend was
certainly in that direction. This might reflect a ceiling effect
for performance. But most importantly, accuracy decreased
significantly between the 20 and 13 c/c distances which
suggests that at 13 mm distance we are honing in on the absolute
threshold. In any case, it is safe to assume that the threshold
falls somewhere below 13 mm.
Comparing the vibro‑sponge and the tactile vest
In experiments 2 and 3, accuracy was higher overall for the
vibro-sponge than the tactile vest, even though the c/c
distance was smaller for the vibro-sponge in both cases. For
example, in experiment 1, accuracy was 0.70 for the vest
(40 mm c/c) but 0.91 with the vibro-sponge (30 mm c/c).
Experiment 2 reveals a similar pattern: the vest with 40 mm
(c/c) distance leads to numerically lower accuracy rates
(p = 0.185) than the sponge with 20 mm (c/c) distance. In
experiment 3 accuracy with the vibro-sponge was
numerically lower than for the vest (p = 0.055), but note that the
inter-tactor distance was 40 mm for the vest but only 13 mm
for the vibro-sponge. It is likely that various hardware
differences between the devices can explain this. The sponge
could be worn tighter to the body, and fit more snugly
against the back than the vest, and the vest may have caused
additional tactile experiences that could add noise during
perceptual judgements. But the most important result from
comparing the two stimulation devices is that performance
was constant between the groups for the tactile vest, while
for the same participant groups, performance on the
vibrosponge decreased significantly as the inter-tactor distance
decreased. This allows us to be confident that individual
differences between the groups do not account for the
decreasing accuracy for the vibro-sponges as a function of distance.
Repetition effects on response times within experiments
While accuracy did not increase throughout experiments (all
p values > 0.26), response times decreased with increased
repetition for the vest (64 tactors; all ps < 0.001) but neither
for the vibro-sponge (all p values > 0.05) nor for the vest
when tested with 9 tactors (p = 0.224; experiment 4, see
below). Stable accuracy and shortened response time do not
suggest speed accuracy trade-offs, but rather that
participants needed less effort to perform the task as the
experiment progressed.
Effects of c/c distance on response times
As for accuracy, we compared effects of distance on response
times between experiments in the combined data set.
Differences in RTs as a function of distance were not far from
being significant in experiment 1 (p = 0.055), but not in the
other experiments (all ps > 0.1; see Fig. 4). The fact that
response times were similar in all experiments shows that
speed/accuracy trade-offs cannot account for the accuracy
differences by c/c distance for the vibro-sponge between
experiments 1, 2 and 3.
Experiment 4—controlling for the area of possible stimulation on the tactile vest
Experiment 4 was a control experiment to check whether
the fact that for the tactile vest the vibration could occur
anywhere within the 8 by 8 tactor grid might account for
decreased accuracy compared to the vibro-sponge where the
stimulation was confined to a 3 by 3 array. We compared
performance on the vest when the stimulation could be
anywhere on the grid during the experiment and when the
possible locus of stimulation was confined to a 3 by 3 grid (at
similar locations on participant’s backs as for the vibro-sponge)
throughout the experiment. Participants in this control
experiment were the same as in experiment 3 and we
therefore used data from experiment 3 (tactile vest, c/c 40 mm). In
experiment 3 the average accuracy was 0.76 (SD = 0.42) and
in experiment 4 it was 0.75 (SD = 0.43). The difference in
accuracy was not significant [F(1,9) = 0.09, p = 0.772]. The
average RT in experiment 3 was 595 ms (SD = 184 ms) and
in experiment 4 it was 649 ms (SD = 289 ms). The
difference (54 ms) was not significant [F(1,9) = 1.25, p = 0.293].
Repetitions neither increased accuracy (p = 0.791) nor
shortened RTs (p = 0.127). Overall, the results show that
performance differences between the tactile vest and the
vibro-sponges cannot be explained by differences in
stimulation area.
Discussion
While a lot is known about the spatial acuity of tactile
perception when it comes to pressure, far less is known about
sensitivity to vibrating stimuli. Here we systematically
investigated the relative spatial acuity of the torso’s skin to
vibrotactile stimulation. Our aim was firstly to gain better
understanding of tactile sensitivity to vibration and secondly
to assist in formulating guidelines for inter-tactor spacing
during the design of tactile displays that use vibrotactile
stimulation. We conducted 3 experiments involving two
types of stimulation devices mounted with coin cell motors.
We gradually decreased the center-to-center inter-tactor
distance from 30 to 13 mm c/c.
Accuracy in all experiments was well above chance
suggesting that the spatial acuity of the torso’s skin is lower
than 13 mm c/c. Accuracy nevertheless dropped
significantly for the 13 mm distance compared to the longer
distances. This result is in line with Eskildsen et al. (1969),
who found a two-point threshold of 10 mm for successive
stimulus presentation on the torso. Even though the
spatial acuity in our study appears higher than in Van Erp
(2005a), who estimated that the spatial acuity across the
torso was 20–30 mm, the difference may partly reflect the
location of the vibro-sponge in our experiment. Since the
vibro-sponge was placed centrally on the back, mainly
covering the spine region, the tactors stimulated an area
for which accuracy has been reported to be higher than
further away from the spine
(Van Erp 2005a)
. Hence, the
threshold found in our study is valid for the center area
of the back and generalizing it to lateral areas should be
done with caution.
Conversely, our results are discrepant with those of
Novich and Eagleman (2015)
who found spatial acuity to
vibrotactile stimulation to be only 60 mm with a tactile vest.
It is interesting to compare their results with our results for
the tactile vest (Fig. 3), since the spatial acuity was
significantly lower than for the vibro-sponge, which probably
involves more direct stimulation than the tactile vest. The
inter-tactor distance was 40 mm for our tactile vest, but
accuracy levels hovered between 70 and 80%, way above chance
level. Note also that the task in Novich and Eagleman was
not fully comparable to ours, but it is unlikely that
superficial task differences can explain the difference in accuracy
estimates.
The main practical conclusion that can be drawn
regarding the design of tactile devices in the torso area, is that
coin cell tactors of 10 mm diameter can be placed as close
as possible (13 mm c/c), for above chance performance,
although our results also suggest that performance drops
a bit at this point. But the person wearing the device will
still be able perceive the tactors individually, which is a key
requirement for successfully conveying information using
a tactile language. Yet another consideration is that above
chance performance may not be a particularly ambitious goal
for conveying information. Our aim was not to determine
accuracy levels necessary for any particular device, so any
such criteria must be set by the required resolution for a
particular device.
Our hope is that our findings may help with designing
vibrotactile equipment in general and sensory substitution
devices more generally. Knowledge of spatial acuity
thresholds is important for optimizing displays to fit the perceptual
characteristics of the torso. The specific interest in the torso
has increased, following recent successes in the application
of vibrotactile torso displays in orientation and navigation
tasks in various contexts
(Van Erp et al. 2004)
. By
presenting a spatio-temporal pattern, these displays can indicate the
direction of drift in a helicopter hover task or the direction of
the next waypoint in a navigation task. The more tactors that
can be placed within a tactile display, the more information
can be conveyed without the need to additionally encode
information by deploying the dimension of time. Therefore,
determining spatial acuity thresholds should lead to more
efficient tactile applications.
When determining the vibrotactile spatial acuity of the
torso’s skin, some possible confounding variables have to
be considered, which may explain the inconsistent findings.
Hence, in the following section, we outline and discuss some
of these issues.
One factor that may influence tactile spatial acuity is the
number of tactors and the distance between them, or in other
words, the size of stimulation area, which can vary greatly
between studies. Eskildsen et al. (1969) tested arrays of
5 × 1, van Erp (2005a) tested arrays of 14 × 1 and 11 × 1
and Van Erp et al. (2005b) tested 8 tactors. Using an array
of 3 by 3 tactors with c/c 60 mm (horizontal and vertical,
Lindeman and Yanagida (2003)
reported absolute spatial
accuracy of 84%.
Jones and Ray (2008)
used an array of 4
by 4 tactors with 60 mm c/c, horizontal and 40 mm
vertical, to test absolute spatial localization and found the
average accuracy across all tactors to be 59%. Further analyses
of the data revealed that horizontal accuracy was 87% but
68% vertically. Accuracy by distance (c/c) was very
similar in Lindeman and Yanagida (2003) and
Jones and Ray
(2008)
although the number of tactors differed
considerably between these experiments (9 versus 16, respectively).
Cholewiak et al. (2004) concluded that the most important
factor for localization accuracy is inter-tactor distance. Their
results show that the effect of number of tactors on
localization is ambiguous. Our results on relative spatial acuity
with point localization suggest that decreasing the size of
the area of vibrotactile stimulation does not influence
thresholds for vibrotactile spatial acuity as there is no significant
drop in accuracy when the same device (the tactile vest) is
used with a 3 by 3 or an 8 by 8 tactor array. The tactile
twopoint threshold could also vary by the tested direction. In
our study, only the horizontal axis was tested, but Van Erp
(2005a) did not find any difference by direction. However,
further studies systematically investigating the influence of
stimulation area size, as well as direction, and comparing the
results for absolute and relative spatial acuity are necessary.
The design of tactile devices can cause variability when
spatial acuity is measured. We found that the vest with
40 mm (c/c) distance led to significantly lower accuracy rates
than the sponge with 30 mm (c/c) distance. The main
difference in the design is that the tactors on the vibro-sponge
are mounted on soft foam and are not covered with fabric.
The foam minimizes the distribution of the vibration from
the tactors probably resulting in more fine-tuned
localization of the vibrations. Van Erp (2005a) attached the motors
directly to the skin using thin double-sided adhesive tape,
whereas the participants in our study wore their own shirts
under the tactile devices. This calls for further investigation
of the influence of design of tactile devices on spatial acuity.
We should note that the spatially static stimulation we
used here may underestimate thresholds.
Vierck and Jones
(1969)
demonstrated that the discrimination of the size of
rounded stimuli (discs) is about ten times better than for
point stimuli and
Jones and Vierck (1973)
found that the
discrimination of line lengths was about two times lower than
for 2PL, suggesting that patterns could provide more
information than point stimulation.
Gibson (1962)
found large
differences between passive tactile perception of a stationary
versus moving stimulus. When a ‘cookie-cutter’ was pushed
onto participants’ palms while remaining otherwise
stationary, identification rates of its pattern were just under 50%
percent, but if the cutter was pushed around in the
observers’ palm, recognition accuracy became about 95%
(see also
Novich and Eagleman 2015)
. Moving patterns may therefore
increase the actual resolution.
Furthermore, the chosen paradigm can influence
spatial thresholds. When comparing PL and 2PL methods,
Weinstein (1968)
found that spatial tactile acuity with the
two-point threshold was 2–4 times lower than with point
localization although both thresholds were highly correlated
(e.g., thresholds for the back were ca. 40 mm for the PL and
10 mm for the 2PT). However, the former method, 2PL,
cannot be applied with tactors, as it requires comparing
the conditions of running either one tactor or two tactors
to find out if the two-tactor condition is perceived as one.
Vibrotactile simulation involves frequencies with
particular phase, which leads to a noticeable phase difference as
soon as two vibrotactile tactors with different phases run
at the same time. These phase differences are clear
indicators of simultaneously running tactors, which participants
can base their decision on. We therefore applied the point
localization paradigm by presenting two successive stimuli
instead of the 2-point-threshold approach. Additionally, it
is important to note that results on relative spatial acuity
as in this study are not directly comparable to
measurements of absolute spatial acuity. The ability to localize a
point of vibrotactile stimulation on the back (absolute) does
not appear to reflect limitations with relative spatial acuity
(Jones 2011)
.
Finally, the high variance in findings on spatial acuity
may stem from different tactor types. The complex nature
of vibro-tactile stimulation renders comparisons of studies
investigating vibro-tactile spatial acuity thresholds difficult.
The physical characteristics of vibrotactile signals itself can
vary by amplitude and frequency although there is as fixed
relationship between frequency and amplitude of ERM
tactors
(Jones 2011; Precision Microdrives 2017)
that are
commonly used and we used in our experiments. Comparisons
across studies using different tactors are further limited by
interactions between vibro-tactile signal dimensions. For
instance, the frequency and amplitude of vibration are not
orthogonal, since changes in the frequency of a
vibrotactile signal can affect their perceived amplitude
(Bolanowski
et al. 1994; Morley and Rowe 1990)
and vice versa
(Verrillo
et al. 1969)
. Spatial acuity thresholds measured with one
tactor type should be cautiously generalized to other tactor
types. In future work, we will therefore assess vibrotactile
spatial acuity with different tactor types.
Compliance with ethical standards
Conflict of interest The authors report no conflict of interest.
Open Access This article is distributed under the terms of the
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(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
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